1. Field of the Invention
This invention relates to a rear projection screen used for a rear projection display.
2.Related Art
A rear projection display as, shown in FIG. 15, comprises a projection tube 1 for projecting optical images, a projection lens 2 for enlarging the optical images projected from the projection tube 1, and a rear projection screen 3 for forming images of the enlarged optical images, and an observer P observes the enlarged optical images projected from the projection tube 1 behind the screen 3 to the screen 3.
As the rear projection screen 3, a two-panel type screen comprised of a Fresnel lens sheet 4 for converging luminous flux projected from the projection tube 1 to the direction of the observer P and a lenticular lens sheet 5 which disperses the light emitted from the Fresnel lens sheet 4 to the horizontal direction of the screen (screen width direction) and the height direction of the screen (screen height direction) at prescribed angles at an appropriate rate for spreading a viewing angle to a prescribed range.
Recently in rear projection displays, decreasing distance between the projection lens 2 and the screen 3 is demanded to make the display slimmer. To meet this demand, it is necessary to increase the Fresnel angle of the Fresnel lens sheet 4. If this Fresnel angle is increased, however, reflection loss at the peripheral area of the Fresnel lens sheet 4 increases and the ratio of luminosity at the corners of the screen 3 to that at the center area of the screen 3 (so called peripheral luminance ratio) drops, which is a problem.
If the peak screen gain is increased to improve the luminosity of the rear projection display, the drop of peripheral luminance is an even more serious problem since the quantity of the diffuser, which is mixed in the component materials of the screen 3 for adjusting the dispersiblity of the light emitted from the screen 3, is decreased.
Conventionally, various countermeasures against such a problem of a peripheral luminance drop have been suggested. For example, the lenticular lens 7 for vertical diffusion is disposed on the opposite side of the Fresnel lens 8 of the Fresnel lens sheet 4 by vertically disposing many unit lenses, which are micro-cylindrical lenses 7u arranged with their longitudinal direction in the horizontal direction, as shown in FIG. 16 (e.g. Japanese Patent Application Laid-Open No. 2-18540). Another method known is increasing the vertical diffusibility of the light of a vertical lenticular lens at the peripheral area more than at the center area of the screen as shown in FIG. 17, so that a drop of the luminance at the peripheral area can be controlled (e.g. Japanese Patent Application Laid-Open No. 7-134338).
Another method is directing the light in the vertical direction to the direction of the observer by combining two linear Fresnel-lenticular lenses (two linear Fresnel lenses which also serve as lenticular lenses) 9a and 9b to improve the luminosity at the peripheral area of the screen as shown in FIG. 18 (U.S. Pat. No. 4,531,812).
Another method is combining two linear Fresnel lenses, one for the vertical direction and the other for the horizontal direction, to deflect the image light to the vertical direction (U.S. Pat. No. 5,477,380).
Another method is using a Fresnel lens sheet where a Fresnel lens is disposed on the incident surface of the light from the projection tube and a linear prism for deflecting light in the vertical direction is disposed on the outgoing surface (U.S. Pat. No. 4,512,631). With the above mentioned method of using the lenticular lens 7 for vertical diffusion in FIG. 16, however, when the peak screen gain exceeds 7, color irregularity (color shift) of the screen deteriorates because the refractive index of material differs depending on the difference of color (difference of wavelengths), therefore, reflection loss on the Fresnel lens differs depending on the difference of color, although color irregularity of the screen is not a problem when the peak screen gain is low due to the contribution of diffusion of lights in the vertical direction of the lenticular lens 7 for vertical diffusion. Another problem is that if diffusibility of the lenticular lens 7 for vertical diffusion is increased to increase peripheral luminance, the screen gain at the screen center area drops.
If a vertical lenticular lens is used where the vertical diffusibility of light is higher at the peripheral area than at the screen center area, both luminosity on the peripheral area of the screen and luminosity on the entire screen can be implemented which is unlike the case of using a vertical lenticular lens where the vertical diffusion of light is uniform, but even in this case at the peripheral area of the screen light L.sub.o which diffuses not only to the direction of the observer P but also to the invalid area at the top and bottom of the screen, as shown in FIG. 17, makes it impossible to improve light utilization efficiency.
In the case of a method in FIG. 18, where two linear Fresnel-lenticular lenses 9a and 9b are combined, loss of light due to reflection is greater than the case of using a circular Fresnel lens, since the refraction at the linear Fresnel-lenticular lenses 9a and 9b is high. Also in the case of a screen using the linear Fresnel-lenticular lenses 9a and 9b, light utilization efficiency in the effective area of the screen degrades due to the diffusion of light.
Even in the screen where two linear Fresnel lenses, one for the vertical direction and the other for the horizontal direction, are combined to deflect the image light in the vertical direction, the loss of light is greater than the case of using a circular Fresnel lens.
In the case of a screen using a Fresnel lens sheet where a Fresnel lens is on the incident surface of light from the projection tube and a linear prism for deflecting light in the vertical direction is on the outgoing surface, disposing the Fresnel lens on the incident surface of the light from the projection tube increases the loss of light.